Prior art teaches various methods for underpinning to strengthen settling foundation, via primarily two types of underpinning, namely the active and passive underpinning pile. Further, most of the methods are adapted for underpinning a wall foundation rather than a typical large pile cap which carries a concentrated load from the structural concrete frame of the building.
In passive underpinning pile, the new pile is inserted adjacent to the existing settling foundation and then is either bonded or connected to the pile cap or footing by means of a cantilever bracket, ties or enlarged footings. The settling foundation is allowed to settle further until the passive underpinning pile becomes loaded gradually with time. Such method has weakness as continuing settlement can occur over a long period especially if the pile is deep. Such use of passive underpinning piles is common as in U.S. Pat. No. 4,834,582 by Roger Bullivant which uses slanted cast in situ concrete piles and support ties beams. UK pat no:—2,047,303 by Leonard Flowerday has a vertical pile but a concrete cantilever corbel to carry the adjacent footing.
In an active underpinning pile, a certain amount of preloading is applied instantaneously upon the underpinning pile to support the weakened foundation, preventing it from any further settlement. An existing method in the prior art uses a system of complicated prefabricated steel brackets in conjunction with an arrangement of screws, rams or hydraulic jacks reacting against the existing weak foundations. In some cases, very cumbersome apparatus consisting of twin jacks or rams are used to react against the newly installed underpinning pile. A further weakness in some systems lies in the use of many complicated small parts, which may deteriorate and thus be unable to carry a large load if any key mechanism is damaged by wear or rust.
U.S. Pat. No. 5,120,163 by Holdeman has an inverted U-shaped coupler adapted to be temporarily secured to the foundation support to allow the hydraulic jack to react between the screw pile and footing. This method has many small screws and many other interconnecting parts, thus is adapted only to carry a smaller load due to the slant screw piles. This feature is also quite similar to U.S. Pat. No. 5,213,448 by Seider which uses a bracket for uplifting the footings.
US Pat no:—2002/0176749 A1 by Provost has an oversized anchor pile driven at a 15 degree slant at the foundation edge and installation of an anchor jack-plate having a collar and gusset attached thereto, slideable upon the screw anchor shaft. The eccentricity of the floating piers 16 reduces the capacity of the pile and is a source of weakness due to shearing in the jack plate and bending in the pile.
U.S. Pat. No. 3,796,055 by Mahony, uses a method and apparatus for lifting the foundation by gripping the sides of a pipe to drive it vertically against the adjacent foundation, in conjunction with jacks at a number of specified stations to lift the foundation. Portions of the buildings foundation are raised to further include the step of dispersing a body of hardenable cementations material outwardly of and below the lower end of the pipe to an enlarged hardened region of subterranean support resting on top of the pipe. This method is adapted for small load transfer as the enlarged cementations material can shear easily.
U.S. Pat. No. 5,433,556 by Freeman III, uses a pier driving assembly which includes a pier driving bracket which drives the pier pipes into the ground. This method is not suitable for underpinning inside the building due to the massive and deep excavation underneath the existing weakened foundations
UK Pat no:—2,190,693 A by Frank Whittaker, uses an underpinning device for a wall structure comprising a bracket member 35 extending from a rectangular or tabular, hollow box-like member 34, locatable over the top of a previously driven support pile. In addition, an adjustable support means 37, 38 is located on the bracket 35. This system of cantilever demands that the pile is capable of overcoming the bending resistance from the bracket cantilever member and thus is anticipated for use with steel pipes or such that can offer corresponding bending resistance. Deep excavation is required underneath the footing to allow access for the placing of jacks or adjustable support means. Also wider and extensive excavations are required apart from the reduced capacity of the pile due to bending.
UK Pat no:—2,150,612 A by Arthur Joseph Steel, uses a system for underpinning a wall foundation comprising driving a plurality of tabular sleeves 13, by using hydraulic jacks, into which concrete is poured and allowed to harden. This is also similar in principle to UK Pat no:—2,024,283 A by John Patrick Jones, except that the pile has a central aperture running therethrough. Both methods involve waiting time for concrete curing and workability problems due to delays, as well as deep and wide excavations.
UK pat no:—2,255,990 by Roger A Bullivant uses a method of supporting the foundation by removing a short length of two or three courses of brick and supporting the building above the removed portion on a joist 14 with a pile connector 16 resting on a jack 28. The jack is stressed and the pile connector slides through the pipe pile and is fixed to it after stressing. This method uses the jack to react against the existing footing and thus only a certain reaction force is available from the old footing. The eccentricity of the pile connector 16 causes a large shearing and bending on the welds connecting the portion of C-channel to the pipe shaft 12. This method again is adapted for a low load as in a wall foundation.
Pending Malaysian patent no:—PI 20005487 filed by the present applicant discloses an underpinning pile system that works rather similarly to the present invention. However, PI 20005487 employs welding rather extensively which results in a slower process, inconsistent quality and costly operation. Apart from those problems, safety is another concern since welding parts may snap and cause serious hazards to workers. Therefore there arises a need for a design that eliminates or reduces the above problems. The design should be easy to handle, fast, efficient and safer than underpinning pile systems of the prior art.